Surfactant boundary lubricant film modified by an amphiphilic diblock copolymer

Influence of surfactants on a pre-adsorbed cationic layer: Removal and modification

Removal of organic species from solid surfaces is a crucial process. The use of oppositely charged surfactants provides a potential method for enhanced removal. Neutron reflectometry has been used to investigate the complex behaviour of a pre-adsorbed and tenacious layer of the cationic surfactant didodecyldimethylammonium bromide (DDAB) on a mica surface, during exposure to different organic species in solution. The anionic surfactant sodium dodecylsulfate (SDS) was shown to be able to remove the cationic layer, but only if anionic micelles were present in solution. To facilitate comparison with the behaviour of a non-ionic surfactant, the direct adsorption of pentaethylene glycol monododecyl ether (C₁₂E₅) to mica was also studied; low surface coverage adsorption was seen at the critical micelle concentration and above. C₁₂E₅ was then found not to remove the cationic layer, but did include into the layer to some degree. The presence of cationic surfactant on the mica was however shown to significantly modify the adsorption behaviour of the non-ionic surfactant.

Soft/Hard-Coupled Amphiphilic Polymer Nanospheres for Water Lubrication

Amphiphilic polymer nanospheres of poly(3-sulfopropyl methacrylate potassium salt- co-styrene) [P(SPMA- co-St)] were prepared by a simple soap-free emulsion polymerization method and used as efficient water lubrication additives to enhance the antiwear behaviors of the Ti₆Al₄V alloy. The monodisperse and flexible P(SPMA- co-St) bicomponent copolymer nanospheres were synthesized with a controllable manner by adjusting the mass fraction ratio of the monomers, with the hydrophobic polystyrene (PSt) as the hard skeletal carrier component and the hydrophilic PSPMA with a hydration layer structure as the soft lubrication layer in the course of friction. The influences of the monomer concentration, the copolymer nanosphere additive content, the load, and the frequency of the friction conditions on their tribological properties were studied in detail, and a probable antiwear mechanism of the soft/hard-coupled copolymer nanospheres under water lubrication was also proposed. The results show that compared with pure PSt, the P(SPMA- co-St) polymer nanospheres exhibited better antiwear property as an additive for water lubrication, and the friction coefficient and the wear volume first decreased and then increased with the increase of the SPMA content, indicating that the hydrophilic SPMA has a significant effect on lubrication properties owing to its hydration performance. Furthermore, with the increase of polymer nanosphere concentration, the friction coefficient and wear amount also decreased to a stable and low value at a saturation concentration of 1 wt %. The flexible polymer nanospheres with a hydrophilic soft SPMA shell and a rigid PSt core exhibited good friction-reduction and antiwear performance as lubrication additives, indicating their promising and potential applications in water lubrication and biological lubrication.

Synergies in lubrication

In living organisms the aqueous medium is used for providing low friction forces. This is achieved by synergistic actions of different biomolecules that together accomplish a high load bearing capacity and sustain an easily sheared water layer.

Specular neutron reflection at the mica/water interface – irreversible adsorption of a cationic dichain surfactant

Neutron reflection from the important mineral mica at the solid/liquid interface is presented here using a new approach – a very thin mica crystal supported on a silicon substrate. This approach avoids the problems of crystal defects and surface undulations that have hindered previous work. The use of mica as a reflectivity substrate is important as it is a model surface, which is atomically smooth with a high structural charge. In this work the mica/water interface is fully characterized. In particular, a characteristic double critical edge is observed, arising from the higher scattering length densities of the mica and D2O subphase relative to the silicon support. The experimental data are modelled using a combined approach: conventional amplitude summation (matrix method) for the thin layers and reflected intensity summation with attenuation terms for the thick layers of mica and hydrocarbon adhesive. Reflection data from the adsorption of the dichain cationic surfactant didodecyldimethylammonium bromide (DDAB) to the surface of muscovite mica from aqueous solution are also presented. It is found that, at twice the critical micelle concentration, a bilayer of DDAB with a thickness of 24 Å is observed, containing essentially no water. Its partial removal by washing and ion exchange is also presented.

Stability and tribological performances of fluid phospholipid bilayers: effect of buffer and ions

We have investigated the mechanical and tribological properties of supported Dioleoyl phosphatidylcholine (DOPC) bilayers in different solutions: ultrapure water (pH 5.5), saline solution (150 mM NaCl, pH 5.8), Tris buffer (pH 7.2) and Tris saline buffer (150 mM NaCl, pH 7.2). Friction forces are measured using a homemade biotribometer. Lipid bilayer degradation is controlled in situ during friction tests using fluorescence microscopy. Mechanical resistance to indentation is measured by force spectroscopy with an atomic force microscope. This study confirms that mechanical stability under shear or normal load is essential to obtain low and constant friction coefficients. In ultrapure water, bilayers are not resistant and have poor lubricant properties. On the other hand, in Tris saline buffer, they fully resist to indentation and exhibit low (micro=0.035) and stable friction coefficient with no visible wear during the 50 min of the friction test. The unbuffered saline solution improves the mechanical resistance to indentation but not the lubrication. These results suggest that the adsorption of ions to the zwiterrionic bilayers has different effects on the mechanical and tribological properties of bilayers: higher resistance to normal indentation due to an increase in bilayer cohesion, higher lubrication due to an increase in bilayer-bilayer repulsion.

Delamination and renovation of a molecular surfactant-polymer boundary lubricant film

We have studied the behavior under compression and shear of two molecularly smooth mica surfaces immersed in aqueous solutions of a dimeric cationic surfactant and an oppositely charged polyelectrolyte-neutral diblock copolymer by using a surface force apparatus-nanotribometer, SFA-N. The surfactant and copolymer coadsorb as a mix molecular boundary lubricant film on the negatively charged mica surfaces leading to low friction in a sliding mechanical contact. However, under fritting conditions, shearing can induce different dynamic transitions of the confined films. Transitions from the initial low friction steady state to new steady states of low or high friction can be induced when the sliding velocity is increased above certain values. These dynamical transitions occur together with thickness reduction of the confined film. A reverse transition to the low friction steady state accompanied by the renovation of the film can be triggered by reintroducing some matter in the contact, via increasing the amplitude of the fritting cycles.

How do silanes affect the lubricating properties of cationic double chain surfactant on silica surfaces?

The effect of an aminosilane on the lubricant properties of a C(18) double-chained cationic surfactant has been investigated in the context of glass fiber forming process. The surfactant adsorption was studied on silica by Fourier transform infrared (FT-IR) spectroscopy in the attenuated total reflexion (ATR) mode as a function of the aminosilane concentration in an organic water based formulation (sizing) used to coat the glass fibers during the process. A reciprocating ball-on-plate tribometer was used to compare friction properties of silica in contact with the aminosilane-surfactant mixture and in presence of each component of the sizing. Surface forces were measured between silica and an atomic force microscope (AFM) silicon nitride tip in the sizing and in the pure cationic surfactant solution. The aminosilane on its own has no lubricant property and reduces or even suppresses the cationic surfactant adsorption on silica. However, the silica-silica contact is lubricated even if the infrared spectroscopy does not detect any surfactant adsorption. The repeated contacts and shear due to the friction experiment itself induce accumulation, organization and compactness of surfactant bilayers.

Reinforcement of a surfactant boundary lubricant film by a hydrophilic-hydrophilic diblock copolymer

The coadsorption from aqueous solutions of an anionic-neutral hydrophilic-hydrophilic diblock copolymer onto a mica-suported surfactant bilayer of a cationic oligomeric surfactant has been investigated. By using an atomic force microscope and a surface forces apparatus nanotribometer, we studied the resulting film morphology, the interactions between two coated surfaces, and the frictional properties of the boundary film. When the coated surfaces were compressed while being fully immersed in an aqueous surfactant solution, the hemifusion of the adsorbed surfactant bilayers could be easily induced. Noticeable friction forces could then be measured between the monolayer-coated surfaces. Coadsorbing poly(acrylic acid)-poly(acrylamide) diblock copolymer with the cationic surfactant changes the cohesion of the adsorbed layers. When the copolymer concentration is sufficiently high, the hemifusion instability of the adsorbed layers can be inhibited, considerably improving its lubricant properties.

Polyisobutylene-block-poly(methacrylic acid) diblock copolymers: self-assembly in aqueous media

Ionic amphiphilic diblock copolymer polyisobutylene-block-poly(methacrylic acid) (PIBx-b-PMAAy), with various lengths of nonpolar (x=25-75) and polyelectrolyte (y=170-2600) blocks, spontaneously dissolve in aqueous media at pH>4, generating macromolecular assemblies, the aggregation number of which depends on external stimuli (pH and ionic strength). Spherical micellar morphology with a compact core formed by the PIB blocks and a swollen corona built up from the PMAA blocks was deduced by cryogenic transmission electron microscopy. The micelles were further characterized by means of dynamic and static light scattering as well as small-angle neutron scattering. The critical micellization concentration, estimated by means of fluorescence spectroscopy with the use of pyrene as a polarity probe, is decisively determined by the length of the PIB block and is insensitive to changes in the length of the PMAA block.

Highly efficient protein separations in capillary electrophoresis using a supported bilayer/diblock copolymer coating

A surfactant/polymer wall coating consisting of the doubly chained cationic surfactant dimethyldioctadecylammonium bromide (DODAB) and polyoxyethylene (POE) 40 stearate is investigated. The coating is formed by simply rinsing a capillary with a solution containing DODAB and POE 40 stearate. The resultant coating is semi-permanent--demonstrating stable electroosmotic flow (EOF) even after a 60 min high pressure rinse with buffer. The EOF (-0.45+/-(0.23) x 10(-4) cm(2) V(-1) s(-1) at pH 7.4) is suppressed by more than a factor of ten compared to that observed for DODAB alone. Model protein mixtures were separated over a pH range of 3-10 with efficiencies of up to greater than 1 million plates/m for the basic proteins cytochrome c, lysozyme, ribonuclease A and alpha-lactalbumin, and the acidic proteins insulin chain A, trypsin inhibitor, and alpha-chymotrypsinogen A. Migration time reproducibility was 0.5-4.0% from run to run and 0.6-4.3% from day to day. Protein recoveries with this coating ranged from 84% to 97%.

Formation of Molecular Bundles from Self-Assembly of Symmetrical Poly(oxyalkylene)−Diamido Acids

Sodium salts of poly(oxypropylene)-trimellitic amido acid (POP-amido acid), prepared from the reaction of POP-diamines and trimellitic anhydride, were found to self-assemble into orderly molecular bundles. The POP-amido acid has a symmetrical structure consisting of a hydrophobic POP middle block (2000 g/mol) and four symmetrical carboxyl end groups. By dissolving in water and evaporating on a polyether sulfone film, the POP-amido acid molecules self-assembled into a unique array with average dimensions of 7-13 nm in width, 2-5 nm in height, and 20-50 nm in length, observed by atomic force microscope. Varied morphologies were also observed when varying the pH, solvents, evaporating rate, concentration, and substrate surface. Unlike the common surfactants of single head-to-tail structure and the naturally occurring phospholipids of one head and two tails, the synthesized POP derivative is a symmetrical structure of four hydrophilic heads and one long hydrophobic block. Through the complementary noncovalent bonding forces, the molecules tend to align into molecular bundles or loops as the primary structure. The formation of different morphologies is controlled by the intermolecular forces including hydrogen bonding, aromatic pi-pi stacking, ionic charge, and hydrophobic interaction, in a concerted manner.